A common problem facing telephone users today is that they are using many different telephones, each with a different directory number, to receive their calls. The problem is compounded by the fact that many of these numbers are connected to dissimilar systems. The classic illustration of this is when a person uses a wireless (or cellular) phone, an office phone, and a home phone. Other parties don't know which number to call that person on to actually get that person immediately. As wireless (or cellular) telephone technology improves, the coverage is getting more comprehensive, the service is becoming more affordable, and the cellular phone is beginning to become the first choice of telephone numbers at which a person can be reached. Unfortunately, it is not the phone number that is published in the Yellow Pages or the White Pages and, therefore, it is not the phone number by which a person is generally identified. It is an unlisted number given by the cellular service company and because of the per-minute cost to the user, the user is very selective about making the number available to others. The problem, then, is to allow a caller to call just one (primary) phone number, the existing one that is published on business cards and in the Yellow Pages or the White Pages, and nonetheless reach the called person on a second phone, such as their cellular phone, as well as on the primary listed phone.
The basic telephone system is designed to route a single call to a single phone number. The telephone switching network can be modified to ring two phone numbers with one call. But, in the industry today, the only way to do that is to have a third number allocated which people then call and, within the complex network of the centralized system, a service control point actually receives that call and then generates two new calls on the network. So, instead of having one call from one person to another, one now has one call to a machine, which then generates two new calls, so that there are three phone calls on the network. All of these network resources are held up while two phones are ringing until one is answered, and then the telephone system reverts back to one caller, one phone number. This method of connection also requires the use of a database to store information pertaining to the called customer and to complete the transaction.
This method is very expensive because of the needed central office equipment. Also, there are standards yet to be written to allow a central service to generate two calls from one. Thirdly, there are costs due to the administration of the database. This follows since the central computer must know the two phone numbers that the incoming call is trying to reach. The incoming call needs to be triggered in such a way that it is not routed directly to either phone number. It must be put on hold and the information routed to the computer to fetch the two needed phone numbers.